Regarding chargers, I have just ordered one from
http://evworks.
a decent price, than the one quoted by James. Its not adjustable for
pack size so you have to match it to your pack, but for me its fine.
I only have one electric vehicle, my boat, and mostly I'll be charging
from the sun., so its not important to be adaptable. I plan to run
this charger from small genset (Honda 1kva) as an emergency backup.
Regarding the issue of BMS, I hve been following the discussion that
James points to and Jack Rickard makes some persuasive points about
not using a BMS. If you do bottom balance manually, maybe you can get
away without using a BMS. In such a case you could use pack voltage as
an indicator that you are approaching empty, and take suitable action
to avoid destroying cells.
I have a BMS, and I'm still learning about it and LFP batteries as I
use them, (and misuse them!), and without it I may well have destroyed
some cells at low voltage. On the other hand, (and it would be good
to follow Jack Rickards discussion on this) if I didn't have a BMS
maybe the cells would never have been out of balance at the bottom
anyway.
When my pack is balanced nicely none of the cell protectors shunt any
power - it only happens when the pack is not balanced and one or more
cells reach the upper voltage limit before the rest of the pack, and
the cell protectors have to burn off some current. I'm using a dc-dc
converter to charge the LFP cells by taking power from the house pack.
The dc converter is set at 61.6 (which should be 3.85 volts per
cell) and as the voltage approached this limit, with a balanced pack,
say around 60 volts, the current naturally drops off to a very low
level.
If you don't have a BMS you'd have to manually balance the pack and
this would mean lots of close attention to cell voltage as the pack is
charging. And without a BMS you'd have to pay close attention to
individual cell voltages as you approach full discharge.
The individual cells have different amphour capacities and so its
natural that the lowest capacity cells will reach either the upper or
lower voltage limit before the other cells in the pack. At the point
when the lowest capacity cell is reaching low voltage (2.5v) the pack
voltage can still be looking good.
Consider this scenario. The pack voltage is 51.2v. All cells are at
3.2 volts. One of the cells is right at the "knee" of the discharge
curve, and so as you suck a few more amps out of the pack its voltage
drops to 2.5. This is only 0.7 volts lower, and so all you see with
the pack voltage is that it drops to 50.5v - not a lot of change, and
can easily go un-noticed. This can happen in a few seconds if you are
drawing high power. If it continues to drop you can destory the cell
in seconds. Without a BMS how would you know the voltage of this cell
is low?
A BMS will shut off the power or sound an alarm to warn of low voltage.
The same can happen at the top of the charge cycle, but not such a
rapid failure because the amps are lower. But again, without a BMS you
have to be eagle-eye and be watching every cell manaully.
The bottom line for me is that a BMS is a good thing. Perhaps later
when I know lots more about these batteries, I might get by without
one. But for now, it helps guard against losing cells due to my lack
of understanding and practise of how to properly manage them.
--Chris
Quoting James Sizemore <james@deny.org>:
> A good multi-stage charge is far far more important then a good BMS,
> There are large debates on more then a few electric car sites that
> say some BMS hurt instead of helping. The
> http://web.me.
> for no BMS at all. Though a good charger is not all that cheap
> either :
> http://www.evcompon
>
> So you will need to pony up the dead presidents either way.
>
> On Dec 25, 2009, at 12:42 PM, Ned Farinholt wrote:
>
>> Does anyone know how important it is to have a Battery Management
>> System on a bank of LiFe PO cells. It adds considerably to the
>> cost. I can understand taking the risk on a bicycle or motorcycle
>> bank but in a boat, is it prudent?
>> Ned
>>
>>
>
>
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